1. Field of the Invention
This invention relates to zoom lenses suited to be used in photographic cameras or video cameras and, more particularly, to a zoom lens which has a front lens unit of negative refractive power, or is of the so-called "negative lead" type, while still permitting minimization of the size of the lens system to be achieved.
2. Description of the Related Art
The zoom lenses of the negative lead type with size at a minimum have been disclosed in Japanese Laid-Open Patent Applications No. Sho 63-25613 and No. Hei 2-238417, for example. Each of these zoom lenses comprises, from front to rear, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power, with zooming being performed by varying the separations between the lens units. The use of such a type has made it possible to realize a minimization of size over the minus-plus form of the 2-unit type zoom lens which had so far been well known.
The zoom lenses each composed of three lens units having negative, positive and negative refractive powers are described also in Japanese Laid-Open Patent Applications No. Sho 63-271214, No. Sho 64-72114 and No. Hei 2-238418 and U.S. Pat. No. 4,936,661.
However, the above-described conventional examples of the minus-plus-minus form of the 3-unit type zoom lens, despite the total length of the entire lens system being shortened from that of the minus-plus form of the 2-unit type zoom lens, have the disadvantage that they are bulky from the point of view of the outer diameter of the lens.
Meanwhile, with the negative lead type of zoom lens it is, in general, relatively easy to increase the maximum image angle. For this reason, it is expected to be used as the lens system for the panorama photography in recent years.
To achieve widening of the image angle to 90.degree. or more in such a manner that the optical performance is maintained well over the entire area of the image frame, however, it is necessary to set forth proper rules of design of the lens units in terms of the refractive power arrangement, etc. If these rules are improper, the variation with zooming of aberrations is caused to increase. The resultant aberrations could not be corrected well even if the number of lens elements is increased, making it difficult to obtain a high optical performance throughout the entire zooming range.
In such zoom lenses, on the other hand, there has been a growing demand for a much desired increase of the zoom ratio. But, to keep the positions of the three lens units at a high accuracy as they move in differential relation to one another, the mounting mechanism for the lens units and the zooming mechanism tend to become complex in structure and large in size.